Ultraviolet-blue-green light-emitting elements by using an InGaN quantum well active layer have been previously realized by nitride semiconductors. The band gap of InGaN can be changed from an ultraviolet region (3.4 eV) to a near-infrared region (0.7 eV) by changing the In composition. At present, as disclosed in, for example, A. Avramescu, T. Lermer, J. Muller, C. Eichler, G. Bruederi, M. Sabathil, S. Lutgen and U. Strauss, Appl. Phys. Exp 3 061003 (2010) (NPL 1), the maximum wavelength of a nitride semiconductor laser is 531 nm (blue-green). Also, Japanese Unexamined Patent Application Publication No. 2009-246005 (PTL 1) discloses a method for improving the light emission characteristics of a semiconductor laser diode in such a long wavelength region by using a ZnO single crystal substrate.